An investigation has been made of the influence of different modification methods (ion exchange, heat and steam treatment and the application of an oxide coating) on the para-selectivity of pentasils in the alkylation of toluene with ethanol, with the use of its weak aqueous solutions modelling the product of fermentation processing of cellulose-containing feedstock. It has been shown that, under conditions of great dilution of the reaction mixture with water, pentasil modified with polymethylsiloxane makes it possible to produce p-ethyltoluene with a selectivity of at least 98% and with a total toluene conversion of 20%.